The present invention relates to a transmission device designed to transmit electrical signals, optical signals, electric power or the like between two relatively rotatable components via a transmission medium provided between inner and outer cases respectively associated with these two components, and more particularly, to a transmission device which makes it possible to increase the possible relative revolution number of the two components without using a longer transmission medium, features high operation reliability, and permits reduction in size.
Conventionally, the signal transmission between two relatively rotatable components, e.g., an automotive steering shaft component as a rotary component and a steering column component as a stationary component, is performed using a rotary connector, that is, a transmission device provided with a spirally wound, flexible flat cable (hereinafter referred to as FFC) and inner and outer cases for receiving it (Japanese provisional utility model publication no. 62-168581). Here, the FFC refers to a band flexible transmission line such as a cable, which comprises a plurality of electric conductors arranged in parallel to each other and coated with an insulating material, or an optical fiber cable which comprises a plurality of optical fibers arranged in parallel and covered with a sheath.
In this type of transmission device both ends of the FFC are fixed to the inner case and the outer case, respectively, so that the FFC is wound or unwound as the two cases rotate relatively to each other. Accordingly, signal transmission between the rotary component and the stationary component can be performed, with the FFC wound or unwound, even while the rotary component, attached to, e.g., the inner case, rotates relative to the stationary component on which the outer case is mounted. For instance, it becomes possible to transmit a horn signal or auto drive signal received from a switch provided on the steering wheel side of a car to a controller or the like on the car body side via a steering column, or to electrically connect an impact sensor and diagnosis unit provided on the car body side of the car equipped with an air bag system to an inflater of the air bag of the steering wheel side.
The applications of the transmission device described above are limited to those wherein the rotary component performs reciprocal rotation, that is, the rotary component alternately revolves clockwise and counterclockwise, relative to the stationary component. This type of transmission device, however, has an advantage in that the absence of slide-contact parts such as a brush and slip ring ensures no noises for signals. It is because such noises would be caused by the sliding motion of a slide-contact part, or by instantaneous interruption of the signals arising from mechanical vibration of the slide-contact part. Therefore, it becomes possible to configure a highly reliable transmission line. On the other hand, the possible number of relative revolutions of the inner case and the outer case is subject to the number of windings of the FFC. Therefore to increase the possible number of revolutions, the number of windings of the FFC must be increased, i.e., the length of the FFC must be increased. As the number of the windings of the FFC is increased, however, it becomes more difficult to transmit the rotary force of the rotary component to the entire FFC. frequently leading to such trouble as uneven movement of the entire FFC when the rotary component turns and the reversing of the FFC which may do harm to the FFC itself: tearing off ultimately.
In the past, therefore, a transmission device was proposed wherein the FFC is arranged so that the winding direction of the FFC on the inner case side is opposite from that on the outer case side, and a U-turn section is formed in the middle of the FFC in order to make it possible to increase the possible number of relative revolutions of the inner and outer cases with a minimum number of windings of the FFC. According to the proposed device, when the inner case is rotated in such a direction relative to the fixed outer case that the FFC is taken up by the inner case as the inner case turns, the U-turn section of the FFC moves around the inner case in the same direction as the rotational direction of the inner case, and the movement of the U-turn section half offsets the rotation of the inner case. In addition, the rotary force of the inner case can be well transmitted to the whole FFC thus permitting the increase of the possible number of revolutions with a minimum number of windings of the FFC.
If, however, the inner case is rotated in the opposite direction from that described above, that is, if the inner case is turned in the direction so that the FFC is unwound from the inner case as the inner case rotates, then the FFC develops a slack, which interferes with the U-turn section smoothly moving in the case-rotating direction. This causes a moving U-turn section to fail to adequately offset the rotation of the case and to sufficiently transmit the rotary force.
Thus, the proposed device discussed above has the disadvantage that, in some direction of the case, the U-turn section fails to move smoothly, disabling the intended effect of increasing the number of case revolutions with a minimized number of windings of the FFC. Further, the proposed device also requires that the case perform reciprocal rotation; therefore, the case must inevitably be turned also in the direction which inconveniences smooth movement of the U-turn section. For this reason, the proposed device has not yet been put into practical use.
Furthermore, the proposed transmission device described above bears a problem in that it is difficult to reduce the size of the device. More specifically, to ensure smooth movement of the U-turn section and smooth winding and unwinding of the FFC. the volume of the FFC receiving space formed by the inner case and the outer case should be set to approximately double the FFC occupying volume which is defined by the length, width and thickness of the FFC. This is because, for instance, if the FFC housing or accepting space volume is too small and the bending or curvature radius of the FFC at the U-turn section is too small, then an excessive stress is applied to the FFC when the U-turn section moves. This causes the electric conductors, optical fibers, etc. constituting the FFC to break off soon, resulting in a shorter service life of the transmission device.
Especially when external equipment to be connected via a transmission circuit has many circuits, the number of the transmission lines constituting the FFC has to be increased with a consequent increase in the width of the FFC. Accordingly, the cases of the transmission device become larger, requiring a larger space for mounting it on a vehicle or the like. Furthermore, the FFC housing space is exclusively used for housing/accepting the FFC. and no components other than the FFC can be placed in the FFC housing/accepting space, making it half a dead space. Additionally, as the width of the FFC increases, an offensive noise produced when the FFC winds or unwinds tends to become larger. For the reasons described above, it is desired that the FFC housing/accepting space be made smaller.